Pressurized water nuclear reactors include, inside a vessel, the core of the nuclear reactor consisting of prism-shaped assemblies arranged with their axes vertical and, above the core, upper internals which include, a set of vertical guide tubes making it possible to guide the control rods consisting of bundles of rods placed parallel to each other and containing material which absorbs neutrons.
The guide tubes for the upper internals include discontinuous guide elements consisting of flat plates, called guide panels which are pierced with openings, and continuous guide means consisting of sleeves and slit tubes placed in the vertical direction of the tube, the openings of the plates and the bores of the continuous guide means being in positions corresponding to the positions of the absorber rods in the bundle of the control rods.
The openings passing through the guide panels and the bores of the sleeves and of the split tubes constituting the guide elements of the absorber rods have lateral slots connecting them with a free bore at the central part of the guide tube and allowing the passage of a spider assembly of the cluster of parallel absorber rods.
In order to control the reactor by adjusting the reactivity of the core, the clusters of absorber rods are moved in the vertical direction, during the operation of the reactor, such that the absorber rods are introduced to a greater or lesser height inside some assemblies of the core. During their movement in the vertical direction, the absorber rods of the control rods are each guided by a series of aligned openings in the guide panels and by the internal bore of a guide sleeve or a split tube.
After some length of operation of the nuclear reactor, the openings in the guide panels and the bores in the sleeves and split tubes may exhibit wear leading, for example, to ovalization of the cross section of the opening or of the bore, so that the absorber rods are no longer guided satisfactorily.
It is therefore necessary to make periodic measurements and checks on the guide elements of the upper internals of the nuclear reactor, in order to determine whether some of these guide elements need to be replaced or repaired.
The checking of the guide elements must be carried out during a period of shutdown for checking, repairing and/or refuelling the nuclear reactor, during which the upper internals are extracted from the vessel and arranged at the bottom of the cavity of the reactor, on a storage and maintenance stand.
It is necessary to make extremely accurate measurements on the guide elements of the upper internals, insofar as the wear on these guide elements requiring repair or replacement is manifested by small differences between the dimensions of the openings or bores of the guide elements in various transverse directions.
It has been proposed applicant's EP-A-0,390,637, to carry out these checks from comparative measurements of capacitance of capacitors consisting of plates distributed around the axis of a sensor which is moved substantially coaxially inside the guide elements. Preferably, the checking device may be in the form of a cluster similar to a control rod cluster which can be moved inside a guide tube. The absorber rods of the control rod cluster are replaced by axially directed cylindrical bars in two parts, between which a measurement probe consisting of plates of capacitors distributed around the axis of the probe is interposed.
Such a device can be carried by a carrying device moving over the bottom of the reactor cavity, below the upper internals, in line with a guide tube. A mast maneuvered from the upper level of the cavity can be attachment spider assembly of the inspection rod cluster, so as to move the inspection rod cluster in the axial direction inside the guide tube, in the vertical direction and upwards.
It is thus possible to check the guide elements for the upper internals without having to insert the checking device through the top of the internals in position on their inspection stand.
However, the accuracy of the checking obtained by using capacitive measurement probes may be insufficient in certain cases and the processing of the measurements may present some difficulties.
In FR-A-92 09915, a checking device was proposed which includes a mast on which pivoting arms are mounted, constituting supports for rotary probes making it possible to successively fit the rotary probes in each of the guide elements to be checked. Such a device is complex and has moving elements which may be damaged during their use in the reactor cavity.